Phosphorylation and regulation of beta-catenin by casein kinase I epsilon.

Chiron Corporation, Horton Street, Emeryville, CA 94608, USA.
beta-Catenin transduces cytosolic signals to the nucleus in the Wnt pathway. The Wnt ligand stabilizes cytosolic beta-catenin protein, preventing its phosphorylation by inhibiting glycogen synthase kinase 3 (GSK3). Serine-33 and -37 of beta-catenin are GSK3 phosphorylation sites that serve as recognition sites for the beta-TRCP-ubiquitin ligase complex, which ultimately triggers beta-catenin degradation. Mutations at those two sites, as well as in Ser-45, stabilize beta-catenin. Recently, casein kinase I epsilon (CKI epsilon) has been shown to be a positive regulator of the Wnt pathway. Its action mechanism, however, remains unknown. Here I show that Ser-45 is phosphorylated not by GSK3 but by CKI epsilon. Axin, a scaffold protein that binds CKI epsilon and beta-catenin, enhances this CKI epsilon-mediated phosphorylation. Overexpression of CKI epsilon in cells increases the amount of beta-catenin phosphorylated at Ser-45. Ser-45 phosphorylated beta-catenin is a better substrate for GSK3, which suggests that CKI epsilon and GSK3 may co-operate in destabilizing beta-catenin. In spite of the fact that CKI epsilon was found as a positive regulator of the Wnt pathway, mutational analysis suggests that mutation of Ser-45 regulates beta-catenin stability by inhibiting the ability of GSK3 to phosphorylate Ser-33 and -37, thereby disrupting the interaction between beta-catenin, beta-TRCP and Axin. I propose that phosphorylation of Ser-45 by CKI epsilon plays an important role in regulating beta-catenin stability.
Mesh Terms:
Animals, Axin Protein, Casein Kinases, Cytoskeletal Proteins, Humans, Mice, Mutation, Phosphorylation, Protein Kinases, Proteins, Proto-Oncogene Proteins, Repressor Proteins, Signal Transduction, Structure-Activity Relationship, Trans-Activators, Wnt Proteins, Zebrafish Proteins, beta Catenin
J. Biochem. Nov. 01, 2002; 132(5);697-703 [PUBMED:12417018]
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